A supercapacitor, which is also known as an ultracapacitor and formerly known as an electric double-layer capacitor (EDLC)) is a high-capacity electrochemical capacitor with capacitance values up to 10,000 farads at 1.2 volt that bridge the gap between electrolytic capacitors and rechargeable batteries. They typically store 10 to 100 times more energy per unit volume or mass than electrolytic capacitors, can accept and deliver charge much faster than batteries, and tolerate many more charge and discharge cycles than rechargeable batteries. They are however 10 times larger than conventional batteries for a given charge.
Supercapacitors are used in applications requiring many rapid charge/discharge cycles rather than long term compact energy storage: within cars, buses, trains, cranes and elevators, where they are used for recovery energy from braking, short-term energy storage or burst-mode power delivery. Smaller units are used as memory backup for static random-access memory (SRAM).
The salient feature of a supercapacitor is its ability to deliver much higher power density than a conventional battery. Supercapacitors are usually large devices owing to specific design requirements. Supercapacitors are constructed with two metal foils (current collectors), each coated with an electrode material such as activated carbon, which serve as the power connection between the electrode material and the external terminals of the capacitor. Specifically to the electrode material is its very large surface area. The activated carbon is electrochemically etched, so that the surface of the material is about a factor 100,000 larger than the smooth surface.
Supercapacitor electrodes are generally thin coatings applied and electrically connected to a conductive, metallic current collector. Electrodes must have good conductivity, high temperature stability, long-term chemical stability (inertness), high corrosion resistance and high surface areas per unit volume and mass. Other requirements include environmental friendliness and low cost.
The amount of double-layer as well as pseudocapacitance stored per unit voltage in a supercapacitor is predominantly a function of the electrode surface area. Therefore supercapacitor electrodes are typically made of porous, spongy material with an extraordinarily high specific surface area, such as activated carbon. Additionally, the ability of the electrode material to perform faradaic charge transfers enhances the total capacitance.
Generally, the smaller the electrode's pores, the greater the capacitance and energy density. However, smaller pores increase (ESR) and decrease power density. Applications with high peak currents require larger pores and low internal losses, while applications requiring high energy density need small pores. Carbon electrodes for use in superconductors have been made with various carbon sources, including activated carbon, activated carbon fibers, carbon aerogel, carbide-derived carbon, graphene sheets, carbon nanotubes, onion-shaped carbon nanoparticles and template carbon.